1. Field of the Invention
The present invention relates to an image evaluation method and a quality control method suitable for a positive planographic printing plate for use with an IR laser, which printing plate is generally called direct plate-making type, capable of direct plate-making from digital signals of computers or the like.
2. Description of the Related Art
In recent years, laser development has been remarkable. Specifically, a solid state laser and a semiconductor laser having a light emitting region in a range from near IR to an IR region have been developed to have high output and small size. Accordingly, as an exposure light source at the time of direct plate making from digital data of a computer or the like, such lasers are extremely useful.
A positive planographic printing plate precursor for IR laser, for which the above-mentioned IR laser having a light emitting region in the IR region is used as an exposure light source, is a planographic printing plate precursor (hereinafter, referred to simply as a PS plate in some cases) that comprises an aqueous alkaline solution-soluble binder resin, an IR dye, which absorbs light and generates heat, and the like as essential components.
When the positive planographic printing plate precursor for IR lasers is exposed with an IR laser beam, in non-exposed portions (i.e., image portions), the IR dye or the like in the photosensitive layer continues to work as a dissolution inhibitor to substantially decrease the solubility of the binder resin, due to the interaction between the IR dye and the binder resin. On the other hand, in exposed portions (non-image portions), the IR dye or the like absorbs light and generates heat, so that the interaction of the IR dye or the like with the binder resin is weakened. Accordingly, at a time of development, the exposed portions (non-image portions) are dissolved in an alkaline developer to produce image portions.
However, as compared with a positive planographic printing plate precursor to be used for plate-making by UV exposure, such a positive planographic printing plate precursor for IR lasers has a narrow latitude with respect to the activity of a developer and therefore, has problems wherein if the activity is too high, the image part density and printing resistance are decreased and if the activity is too low, development failure easily occurs.
The above-mentioned problems are attributed to the following basic differences in the plate-making mechanisms between the above-mentioned positive planographic printing plate precursor for IR lasers and the positive planographic printing plate precursor for plate-making by UV exposure.
The photosensitive layer of the positive planographic printing plate precursor for plate-making by UV exposure comprises an aqueous alkaline solution-soluble binder resin and an onium salt or quinone diazide compound as essential components. When such a positive planographic printing plate precursor is exposed, the onium salt or quinone diazide compound works in the non-exposed portions (image portions) as a dissolution inhibitor similar to the case of the positive planographic printing plate precursor for IR lasers. However, unlike the case of the positive planographic printing plate precursor for IR lasers, in the exposed portions (non-image portions), it is decomposed by light, and generates an acid, and consequently works as a dissolution promoting agent for the binder resin. Accordingly, in the positive planographic printing plate precursor for plate making by UV exposure, the difference in the solubility with respect to an alkaline developer between the exposed portions and the non-exposed portions is very large.
In contrast, with respect to the positive planographic printing plate precursor for IR lasers, although the interaction between the IR dye and the binder resin is weakened in the exposed portions (non-image portions) at a time of exposure, the IR dye does not work as the dissolution promoting agent for the binder and therefore, the difference in the solubility between the non-exposed parts and the exposed parts is small.
Due to the above-mentioned reasons, when a planographic printing plate precursor such as a positive planographic printing plate precursor for IR laser having a narrow latitude to the activity of a developer is used, it is very difficult to control the plate making process to continuously form stable images.
Generally, in the case of developing the positive planographic printing plate precursor for IR lasers, an automatic developing apparatus having a replenishing mechanism for keeping the developer sensitivity constant as much as possible is employed. The replenishing mechanism is for adding a highly active replenisher so as to prevent pH decrease and deterioration of the developability of the developer due to developing of the plates or CO2 absorption.
Practically, in a treatment system for a regular PS plate, the proposed examples of the replenishing mechanism include: a method of adding a replenisher so as to keep conductivity constant; and a method of adding a predetermined amount of a replenisher periodically after the number of plates developed from planographic printing plate precursors reaches a predetermined number or after a predetermined treatment time passes.
However, in the method for controlling a replenisher based on the conductivity, there arises a problem in that pH values vary (thus developability also changes), even at the same condctivity, between the starting time of development and a later stage, because significantly amounts of compositions of a photosensitive layer are dissolved into the developer as the number of the developed plates increases.
Further, in the method of adding a predetermined amount of the replenisher periodically or according to the number of the developed plates, as the amount of the (applied) replenisher per unit surface area of a plate is fixed, the method cannot address to changes such as: the amount of compositions of the photosensitive layer dissolved into a developer varying in accordance with the area of image portions; and the CO2 absorption amount changing over time depending on the installation environment (temperature, humidity, CO2 concentration and the like) of the automatic developing apparatus. Thus, it is difficult to continuously keep the constant developability by control with such a method of periodically adding a predetermined amount of the replenisher.
In addition, there is also known a quality-control method of: obtaining the clear sensitivity of an image formed by developing a positive planographic printing plate precursor for infrared laser with a standard developer; obtaining the clear sensitivity of an image formed by developing the same plate with a worn-out developer; and adjusting plate-making conditions based on the comparison of the two results of the clear sensitivity. However, there remains room for improvement with respect to production of high-quality printed matter such as art books, car catalogues, and the like, and high-definition printed matter such as FM and AM screens having a density of 300 lines/inch (2.54 cm) or higher (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2001-13692).
Generally, in a widely used planographic printing plate precursor using UV exposure, since the precursor has a wide latitude with respect to the activity of a developer, the fluctuation of the developability as described above is not a major problem. However, as described above, since the positive planographic printing plate precursor for IR lasers has a narrow latitude with respect to the activity of the developer, the image formability, especially the dot formability, fluctuates considerably depending on changes in the activity of the developer, which results in problems regarding quality of the planographic printing plate.
Currently, however, no method has yet been discovered that can be applied to general-purpose positive-type planographic printing plate precursors for infrared laser to effectively prevent fluctuation in the image portion area and peripheral length of very fine dots by means of simplified process control.